Fixation of carbon dioxide and related small molecules by a bifunctional frustrated pyrazolylborane Lewis pair

Article abstract of DOI:10.1039/c2dt30448a

The bifunctional frustrated Lewis pair 1-[bis(pentafluorophenyl)boryl]-3,5- di-tert-butyl-1H-pyrazole (1) was employed for small molecule fixation by reaction with carbon dioxide, paraformaldehyde, tert-butyl isocyanate, tert-butyl isothiocyanate, methyl isothiocyanate and benzonitrile, affording the adducts 3-8 as zwitterionic, bicyclic boraheterocycles. Treatment of 1 with tert-butyl isocyanide gave the isocyanide-borane complex 9, whereas the zwitterionic alkynylborate 10 was formed by C-H bond activation of phenylacetylene. The molecular structures of all products 3-10 were established by X-ray diffraction analyses. DFT calculations at the M06-2X/6-311++G(d,p) level of theory revealed that CO₂ fixation by 1 and formation of the adduct 3 is strongly exothermic and proceeds with a low energy barrier of approximately 7.3 kcal mol^-1via an intermediate van der Waals complex.

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PAPER
Fixation of carbon dioxide and related small molecules by a bifunctional
frustrated pyrazolylborane Lewis pair†
Eileen Theuergarten, Janin Schlösser, Danny Schlüns, Matthias Freytag, Constantin G. Daniliuc,
Peter G. Jones and Matthias Tamm*
Received 24th February 2012, Accepted 17th April 2012
DOI: 10.1039/c2dt30448a
The bifunctional frustrated Lewis pair 1-[bis(pentafluorophenyl)boryl]-3,5-di-tert-butyl-1H-pyrazole (1)
was employed for small molecule fixation by reaction with carbon dioxide, paraformaldehyde, tert-butyl
isocyanate, tert-butyl isothiocyanate, methyl isothiocyanate and benzonitrile, affording the adducts 3–8 as
zwitterionic, bicyclic boraheterocycles. Treatment of 1 with tert-butyl isocyanide gave the isocyanide–
borane complex 9, whereas the zwitterionic alkynylborate 10 was formed by C–H bond activation of
phenylacetylene. The molecular structures of all products 3–10 were established by X-ray diffraction
analyses. DFT calculations at the M06-2X/6-311++G(d,p) level of theory revealed that CO2 fixation by 1
and formation of the adduct 3 is strongly exothermic and proceeds with a low energy barrier of
approximately 7.3 kcal mol−1 via an intermediate van der Waals complex.
ortho-phenylene-bridged
2,2,6,6-tetramethylpiperidinoborane
Introduction
TMP-CH2-C6H4-B(C6F5)2 represent the most prominent
examples,13,14 and it was shown that both FLP systems can acti-
vate H2 and even act as metal-free catalysts for the hydrogen-
ation of imines and other substrates.15 Similarly, we were able to
show that the pyrazolylborane 1, in agreement with theoretical
predictions for related molecules,16 is able to promote heterolytic
dihydrogen cleavage under ambient conditions; the resulting
zwitterionic pyrazolium–hydroborate 2 shows potential for cata-
lytic imine reduction (Scheme 1).17 In continuation of this work,
we decided to investigate the fixation of other small molecules
by FLP 1, since in particular Erker’s and related intramolecular
P/B systems exhibit a rich chemistry towards substrates such
as olefins,18 alkynes,18,19 carbon dioxide,20,21 isocyanates,20,22
azides,22 and singlet dioxygen.23 In addition, CO2 fixation and
activation of other small molecules was also reported for several
geminal P/B- and P/Al-based systems in which the spatial
arrangement of the donor and acceptor sites is similar to that
in 1.24,25
Research on so-called “frustrated Lewis pairs” (FLPs) has
witnessed a tremendous increase during the last six years1 since
Stephan’s first report on the ability of the intramolecular
phosphine–borane FLP system para-(Mes)2P(C6F4)B(C6F5)2
(Mes = 2,4,6-trimethylphenyl) to cleave dihydrogen under
ambient conditions.2 This unique reactivity results from the pre-
vention of classical adduct formation by use of sterically
demanding Lewis acids and bases,3 and numerous FLP systems
have been developed to date by combination of bulky P- or
N-donor Lewis bases with polyfluorinated boranes.4,5 In
addition, C-donor Lewis bases such as N-heterocyclic carbenes
and related species have been employed in combination with
B(C6F5)3,6–8 and also alternative Lewis acids such as alanes,9
silylium ions,10 or metal-based systems have been used in place
of boranes.11
For most FLP systems, small molecule activation proceeds in
an intermolecular fashion, whereas a significantly smaller
number of bifunctional P/B and N/B combinations is known
in which the active centres are located close to each other and
are therefore able to act simultaneously and intramolecularly
on other molecules.12–14 Among these, Erker’s ethylene-bridged
phosphinoborane (Mes)2P-CH2CH2-B(C6F5)2 and Rieger’s
It should be emphasized that CO2 fixation by intramolecular
boron–nitrogen FLP systems has not been previously reported,
except for the reaction of CO2 with four-membered boron amidi-
nates by insertion into one of the B–N bonds.26 Therefore, study-
ing the reactivity of 1 towards CO2 and related substrates is of
Institut für Anorganische und Analytische Chemie, Technische
Universität Carolo-Wilhelmina, Hagenring 30, D-38106 Braunschweig,
Germany. E-mail: m.tamm@tu-bs.de; Fax: +49 531-391-5309;
Tel: +49 531-391-5387
†Electronic supplementary information (ESI) available: Details of the
electronic structure calculations and presentations of all calculated struc-
tures together with Cartesian coordinates of their atomic positions.
CCDC 869097–869104. For ESI and crystallographic data in CIF or
other electronic format see DOI: 10.1039/c2dt30448a
Scheme 1 Intramolecular heterolytic dihydrogen cleavage promoted
by FLP 1.
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substantial interest, particularly in view of its potential for the
development of non-metal-mediated procedures for the selective
hydrogenation or hydrosilylation of CO2, as was recently demon-
strated for the intermolecular B/N combination 2,2,6,6-tetra-
methylpiperidine/B(C6F5)3.27,28 Accordingly, we report herein
the reactions of 1 with the small molecules carbon dioxide and
formaldehyde and also with organic isocyanates, isothiocyanates,
nitriles, isonitriles and alkynes.
Results and discussion
Carbon dioxide fixation
Fig. 1 ORTEP diagram of 3 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
B–O1 1.5063(18), B–N1 1.5946(18), B–C13 1.624(2), B–C19 1.631(2),
N1–C1 1.3404(18), N1–N2 1.3735(15), N2–C3 1.3667(17), N2–C12
1.4428(17), O1–C12 1.3067(16), O2–C12 1.1936(16); O1–C12–O2
128.19(13), C12–O1–B 114.90(11), O1–B–N1 98.05(10), O1–B–C13
109.71(11), N1–B–C13 110.21(11), O1–B–C19 106.28(11), N1–B–C19
113.57(11), C13–B–C19 117.15(11).
A pale yellow toluene solution of the pyrazolylborane 1 was
exposed to carbon dioxide gas at room temperature, immediately
affording a colourless solution. After stirring for 10 minutes,
evaporation and rinsing with pentane gave the zwitterionic
carbon dioxide adduct 3 as a white solid in 90% yield
(Scheme 2). Whereas 1 shows only one set of resonances for the
tBu groups in the 1H NMR spectra because of rapid exchange of
the B(C6F5)2 group between the two nitrogen atoms,17 3 gives
rise to two separate signals at 1.06 and 1.45 ppm. The 11B NMR
resonance is shifted from +43.1 in 1 to +2.3 ppm in 3, which
indicates a transition from a trigonal planar towards a tetrahedral
coordination sphere of the boron atom. The 13C NMR signal of
the quaternary carbon atom of the fixed CO2 molecule appears at
144.6 ppm, which is in agreement with the resonance reported
for a six-membered heterocycle formed by insertion of CO2 into
the B–N bond of a boron amidinate.26 The observation of one
19F resonance each for the ortho-, para-, and meta-fluorine
atoms at −135.5, −154.9 and −163.5 ppm, respectively, reveals
free rotation around the B–C bonds at room temperature on
the NMR time scale. In the IR spectrum, the CO stretching
frequency is found in the expected range at 1803 cm−1
.
The molecular structure of 3 was established by X-ray diffrac-
tion analysis (Fig. 1), confirming the formation of the bicyclic
product 3 with two almost coplanar five-membered heterocycles.
The boracycle formed by CO2 addition displays a comparatively
high degree of asymmetry with markedly different intraring
bond distances and angles, displaying for instance a small N1–
B–O1 angle of 98.05(10)° next to a rather obtuse B–O1–C12
angle of 114.90(11)°. The B–O1 bond length of 1.5063(18) Å is
similar to that reported for the six-membered CO2-boron amidi-
nate adduct [1.493(5) Å] (vide supra),26 whereas significantly
longer B–O bonds were found for CO2 adducts of the intra-
moluclar P/B systems (tBu)2P-CH2-BPh2 [1.5645(15) Å]20 and
(Mes)2P-CH2CH2-B(C6F5)2 [1.550(4) Å].21 In agreement with
the previously described CO2 adducts, the C12–O2 and C12–O1
bond lengths of 1.1936(16) and 1.3067(16) Å exhibit the
expected predominantly double- and single-bond character,
respectively.
Fig. 2 Relative energies (kcal mol−1) for stationary points on the 1 +
CO2 hypersurface. Values correspond to ΔE = zero-point uncorrected
M06-2X/6-311++G(d,p) electronic energies. PLUTO drawings of the
van der Waals complex (vdW) and the transition state (TS). Selected
bond lengths (Å) and angles (°): vdW: B–O1 2.814, N2–C12 2.909,
C12–O1 1.158, C12–O2 1.154, O1–C12–O2 177.4; TS: B–O1 1.954,
N2–C12 2.266, C12–O1 1.192, C12–O2 1.149, O1–C12–O2 164.2.
The CO2 adduct 3 was thermally stable under vacuum at elev-
ated temperature and showed no sign of CO2 release. This irre-
versible carbon dioxide binding was further investigated by DFT
calculations; the optimized structures of 1 and 3 agree well with
data derived from X-ray diffraction analysis (vide supra; see
ESI† for a presentation of all calculated structures).17 The for-
mation of 3 was calculated to be strongly exothermic (ΔE =
−29.8 kcal mol−1), which is in agreement with the experimen-
tally observed irreversible CO2 fixation (Fig. 2). Similarly, the
Scheme 2 Carbon dioxide fixation by FLP 1.
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Fixation of isocyanates and isothiocyanates
structurally related adduct isolated from (tBu)2P-CH2-BPh2
showed no release of CO2, and calculations at a similar level
of theory also revealed a high stability of this adduct (ΔE =
−20.3 kcal mol−1).20 In contrast, CO2 fixation with
(Mes)2P-CH2CH2-B(C6F5)2 was calculated to be almost thermo-
neutral (ΔE = −2.9 kcal mol−1), in accord with the experimen-
tally observed reversibility.21
To investigate the fixation of other small molecules that are
(valence) isoelectronic with carbon dioxide, 1 was also reacted
with organic isocyanates and isothiocyanates, which can poten-
tially lead to different regioisomers by N or O/S coordination to
the Lewis acidic boron site. Treatment of a toluene solution of 1
with tert-butyl isocyanate (tBuNCO) afforded the adduct 5 as a
white solid in 76% yield (Scheme 4). The 1H NMR spectra indi-
cated the formation of a new product by showing three signals
for the tBu groups at 0.83, 1.39 and 1.44 ppm; two-dimensional
NMR spectroscopy (HMBC experiment) allowed the assignment
of the resonance at 1.39 ppm to the tBu group of the inserted iso-
cyanate. Similarly to the formation of 3 and 4, the 11B NMR
peak is shifted up-field to 2.5 ppm upon isocyanate coordination.
Finally, the 13C NMR resonance of the quarternary carbon atom
of the NCO moiety is found at 139.8 ppm.
By analogy to the reaction pathways theoretically derived for
these P/B Lewis pairs and also for a related geminal P/Al-based
FLP,20,21,25b our calculations revealed the occurrence of an inter-
mediate van der Waals complex (vdW, ΔE = −7.2 kcal mol−1
)
with an almost linear CO2 molecule (177.4°). The B–O1 and
N2–C12 distances of 2.814 and 2.909 Å in this complex become
progressively shorter upon conversion into 3 (B–O1 = 1.503 Å,
N2–C12 = 1.455 Å, O1–C12–O2 = 129.3°) via a transition state
(TS, ΔE = +0.14 kcal mol−1) that lies only 7.3 kcal mol−1 above
the van der Waals complex (vdW). The resulting B–O1
(1.954 Å) and N2–C12 (2.266 Å) bonds are long and—together
with short C–O bonds of 1.182 (C12–O1) and 1.149 Å
(C12–O2) and a large O–C–O angle of 164.2°—correspond to
an “early” transition state. The reverse reaction and release of
CO2 from 3 requires an activation energy of approximately
30 kcal mol−1, which is consistent with the experimentally
observed irreversibility of the CO2 fixation (vide supra).
X-Ray diffraction analysis allowed the determination of
the molecular structure of 5 (Fig. 4), revealing the incorporation
of the isocyanate CO group into the five-membered ring.
Fixation of formaldehyde
Since formaldehyde represents a possible intermediate during
non-metal mediated homogeneous hydrogenation of CO2 to
CH3OH,27 we attempted the fixation of this small molecule by 1.
Hence, a toluene solution of 1 was treated with an excess of para-
formaldehyde (C3H6O3). The pale yellow solution became col-
ourless after stirring the mixture for one hour at room
temperature, and the bicyclic product 4 was isolated in almost
quantitative yield as a white solid after evaporation and extrac-
tion with pentane (Scheme 3). The 11B NMR resonance at
2.5 ppm appears in the same high-field range as observed for 3
(vide supra). The signals for the CH2 moiety are found at 5.56
and 78.6 ppm in the 1H and 13C NMR spectra, respectively.
Fig. 3 ORTEP diagram of 4 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
B–O 1.4769(14), B–N1 1.6087(14), B–C19 1.6293(17), B–C13 1.6429
(17), N1–C1 1.3488(14), N1–N2 1.3617(13), N2–C3 1.3460(14),
N2–C12 1.4793(13), O–C12 1.3865(13); O–B–N1 98.57(8), O–B–C19
106.67(9), N1–B–C19 112.72(9), O–B–C13 108.48(9), N1–B–C13
111.40(9), C19–B–C13 117.13(9), C12–O–B 109.65(8).
1
Similarly to 3, two separate H NMR resonances are observed
for the tBu groups at 0.97 and 1.29 ppm, and the 19F NMR spec-
trum is also almost identical.
Crystals of 4·1/2(toluene) suitable for X-ray diffraction analy-
sis were obtained from a mixture of toluene–hexane at −30 °C,
and the molecular structure of 4 is shown in Fig. 3. The five-
membered boracycle adopts an envelope conformation, whereby
the oxygen atom lies 0.46 Å out of the B–N1–N2–C12 plane;
the resulting B–O bond of 1.4769(14) Å in 4 is slightly shorter
than in 3 [1.5063(18) Å]. The bond angles at the boron atom
range from 98.57(8)° (N1–B–O) to 112.72(9)° (C13–B–C19),
indicating a distorted tetrahedral coordination sphere.
Scheme 3 Fixation of formaldehyde by FLP 1.
Scheme 4 Fixation of isocyanates and isothiocyanates by FLP 1.
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Fig. 4 ORTEP diagram of 5 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
B–O 1.4937(19), B–N1 1.5924(19), B–C23 1.628(2), B–C17 1.631(2),
N1–C1 1.3386(18), N1–N2 1.3742(16), N2–C3 1.3637(18), N2–C12
1.4478(18), N3–C12 1.2499(19), O–C12 1.3254(17); O–B–N1 98.26
(11), O–B–C23 105.31(12), N1–B–C23 113.88(12), O–B–C17 108.80
(12), C1–N1–N2 108.03(11), C1–N1–B 144.02(12), N2–N1–B 107.95
(11), N1–N2–C12 110.53(11), C12–O–B 115.10(11), C12–N3–C13
120.75(13).
Fig. 5 ORTEP diagram of 6 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
S–C12 1.7533(11), S–B 1.9479(12), B–N1 1.5744(14), B–C23 1.6280
(16), B–C17 1.6287(16), N1–C3 1.3445(14), N1–N2 1.3807(12), N2–
C1 1.3737(14), N2–C12 1.4560(14), N3–C12 1.2518(15), N3–C13
1.4879(15); C12–S–B 94.84(5), N1–B–C23 112.72(9), N1–B–C17
109.33(9), C23–B–C17 119.06(9), N1–B–S 100.54(7), C23–B–S
104.79(7), C17–B–S 108.57(7), C3–N1–N2 108.10(8), C12–N3–C13
125.19(10).
Accordingly, the overall structural parameters of the bicyclic
system are very similar to those established for the CO2 adduct
3, with the newly formed B–O and N2–C12 bonds of 1.4937
(19) and 1.4478(18) Å being almost identical to those in 3. The
exocyclic imine moiety exhibits a very short C12–N3 bond of
1.2499(19) Å in agreement with the presence of a C–N double
bond. The C–N–C angle at N3 is 120.75(13)°, and the tBu
group at N3 adopts an E-configuration with respect to the pyra-
zole ring.20,22
and 1.7533(11) Å are longer than the corresponding B–O and
C–O bonds in 5, affording a B–S–C12 angle of 94.84(5)° that is
smaller than the B–O–C12 angle of 115.10(11)° in 5.
The molecular structure of 6 revealed an apparent mismatch of
the soft Lewis basic sulfur atom with the hard Lewis acidic
boron atom, which might be a result of the steric congestion at
the nitrogen atom in tBuNCS. Therefore, the reaction of 1 with
methyl isothiocyanate was additionally investigated, which
might favour B–N bond formation. Indeed, the 13C NMR reson-
ance of the quarternary NCS carbon atom is observed at
171.3 ppm at significantly lower field than the corresponding
resonance in 6, which indicates the presence of a thiocarbonyl
moiety and formation of the bicyclic boratriazole 7 (Scheme 4).
The reaction of 1 with isothiocyanates was also studied, since
these molecules exhibit two distinctly different binding sites
according to Pearson’s acid–base concept and could therefore
bind to the hard boron Lewis acid either through the soft sulfur
atom or through the hard, but sterically more encumbered nitro-
gen atom.30 Addition of tert-butyl isothiocyanate to a toluene
solution of 1 afforded a white solid in almost quantitative yield
after stirring for 15 minutes and work-up by evaporation and
washing with pentane (Scheme 4). Again, NMR spectroscopy
1
The 11B NMR shift is detected at −1.8 ppm, and the H NMR
spectrum exhibits characteristic singlets at 0.71, 1.63 and
3.09 ppm for the tBu and CH3 groups, respectively.
1
indicates the formation of a single addition product, with the H
The formation of 7 was confirmed by X-ray diffraction analy-
sis (Fig. 6); the N2–C12 and B–N3 bond lengths are 1.4410(15)
and 1.5525(16) Å, with the latter being slightly shorter than the
B–N bond reported for isocyanate adduct of a geminal P/B fru-
strated Lewis pair [1.580(5) Å].24b The exocyclic C–S bond
length of 1.6507(12) Å falls in the range (mean distance =
1.671 Å) established for compounds of the type (X)2–CvS (X =
C, N, O, S).31
NMR resonances of the tBu groups of the pyrazole ring at 0.82
and 1.39 ppm and that of the additional tBu group at 1.33 ppm.
At −3.3 ppm, the 11B NMR resonance appears at higher field
compared to the corresponding isocyanate adduct 5. In the 13C
NMR spectrum, the resonance of the quaternary carbon atom of
the NCS moiety is found at 146.2 ppm.
Single crystals of 6 could be obtained from a toluene–hexane
solution at −30 °C, and X-ray diffraction analysis confirmed the
formation of the bicyclic addition product with incorporation of
the CS fragment into the five-membered boracycle (Fig. 5).
Compared to the isocyanate congener 5, introduction of the
sulfur atom leads to a stronger deviation from planarity and
to a noticeable distortion towards an envelope conformation,
whereby the sulfur atom lies 0.24 Å out of the B–N1–N2–C12
plane. Clearly, the B–S and C12–S bond lengths of 1.9479(12)
Fixation of nitriles and isonitriles
The reactivity of the FLP 1 towards organic nitriles and isoni-
triles was also studied in order to investigate potential BN
addition across the R–CuN or R–NuC moieties. Hence,
addition of benzonitrile (PhCN) to a toluene solution of 1
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Fig. 6 ORTEP diagram of 7 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
S–C12 1.6507(12), B–N3 1.5525(16), B–N1 1.5851(16), B–C14 1.6369
(17), B–C20 1.6400(17), N1–C1 1.3445(15), N1–N2 1.3868(13), N2–
C3 1.3735(15), N2–C12 1.4410(15), N3–C12 1.3289(15), N3–C13
1.4513(15); N3–B–N1 95.85(8), N3–B–C14 112.66(10), N1–B–C14
109.13(9), N3–B–C20 107.03(9), N1–B–C20 114.41(10), C14–B–C20
116.00(10), C13–N3–B 124.81(9), C12–N3–B 115.96(10).
Fig. 7 ORTEP diagram of 8 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°): N1–
C3 1.343(2), N1–N2 1.3842(17), N1–B 1.606(2), N2–C1 1.375(2), N2–
C12 1.477(2), N3–C12 1.255(2), N3–B 1.529(2), C1–C2 1.376(2),
C12–C13 1.487(2), B–C19 1.624(4), B–C25 1.634(2); C3–N1–N2
108.32(12), C3–N1–B 143.79(13), N2–N1–B 107.73(11), C12–N3–B
111.49(13), N3–C12–N2 113.89(14), N3–C12–C13 122.08(14), N3–B–
N1 99.70(12), N3–B–C19 102.5(2), N1–B–C19 115.8(2), N3–B–C25
111.50(13), N1–B–C25 109.32(13), C19–B–C25 116.4(2).
bond of 1.588(5) Å and a C–N bond length of 1.136(4) Å con-
sistent with a triple bond,22 whereas the C12–N3 distance in 8
[1.255(2) Å] corresponds to a double bond.
In contrast to the formation of a five-membered ring by 1,2-
addition of 1 across the C–N bond in benzonitrile (vide supra),
the interaction of 1 with isonitriles could be expected to afford a
strained four-membered ring by 1,1-addition to the terminal
carbon atom.32 However, addition of tert-butyl isocyanide to a
toluene solution of 1 gave a white solid in almost quantitative
yield, which showed a strong absorption at 2302 cm−1 in the IR
spectrum, indicating the presence of a boron-bound isocyanide
ligand. Similarly, a CN stretching frequency of 2287 cm−1
was reported for the P/B system (C6H2Me3)2P-CH2CH2-
B(C6F5)2(CNtBu), in which the isocyanide ligand interacts
solely with the boron atom.22 Formation of the adduct 9 is also
confirmed by NMR spectroscopy (Scheme 5), e.g. by obser-
Scheme 5 Nitrile and isonitrile fixation by FLP 1.
1
afforded a white solid in 85% yield. The signals in the H and
13C NMR spectra are broadened, indicating a dynamic process
in solution. A 13C NMR resonance at 133.4 ppm can tentatively
be assigned to the quarternary nitrile carbon atom, which is at
lower field compared to the chemical shift (121.1 ppm) reported
for the pivalonitrile adduct (Mes)2P-CH2CH2-B(C6F5)2(NCtBu),
which shows no interaction between the Lewis basic phosphorus
atom and the boron-bound nitrile molecule in the solid state.22
Accordingly, 1,2-addition of 1 across the CN bond and
formation of the boratriazole 8 could be expected (Scheme 5),
which was unambiguously confirmed by X-ray diffraction
analysis.
1
vation of three singlets in the H NMR spectrum at 0.93, 0.98
and 1.42 ppm for the three different tBu groups. The 11B NMR
resonance is found at −9.0 ppm, whereas no 13C NMR signal
could be observed for the boron-bound carbon atom.
Suitable crystals of 9·1/2(toluene) for X-ray diffraction analy-
sis were grown from a toluene solution at −30 °C, and the mol-
ecular structure of 9 reveals a tBuNC ligand that is coordinated
in a linear fashion [B–C12–N3 = 173.1(2)°, C12–N3–C13 =
177.5(2)°] (Fig. 8). The B–C12 and C12–N3 bond lengths of
1.641(3) and 1.141(3) Å fall in the range established for related
isocyanide–borane adducts.22,32–34
The molecular structure of 8 is shown in Fig. 7; the B–N3
bond in 8 [1.529(2) Å] is slightly longer than the corresponding
B–N bond in the six-membered boron amidinate–acetonitrile
adduct [1.5147(11) Å],26 but shorter than the B–N3 bond in 7
[1.5525(16) Å], which contains a nitrogen-bound methyl group
(vide supra). The pivalonitrile adduct (Mes)2P-CH2CH2-
B(C6F5)2(NCtBu), in which the nitrile interacts only with the
Lewis acidic boron site, exhibits a significantly longer B–N
Fixation of phenylacetylene
The reactivity of FLPs with terminal alkynes was shown
to proceed either proceed via terminal C–H activation or via
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Fig. 8 ORTEP diagram of 9 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
B–N1 1.534(3), B–C17 1.637(3), B–C23 1.641(3), B–C12 1.641(3),
N1–C1 1.382(3), N1–N2 1.386(2), N2–C3 1.326(3), N3–C12 1.141(3),
N3–C13 1.468(3); N1–B–C17 115.92(18), N1–B–C23 114.89(19),
C17–B–C23 114.71(18), N1–B–C12 102.41(18), C17–B–C12 103.48
(18), C23–B–C12 102.77(17), C1–N1–N2 109.48(17), C1–N1–B
138.86(18), C12–N3–C13 177.5(2).
Fig. 9 ORTEP diagram of 10 with thermal displacement parameters
drawn at 50% probability. Selected bond lengths (Å) and angles (°):
C12–C13 1.2065(16), C13–C14 1.4376(15), B–C12 1.5921(15), B–N1
1.6229(14), B–C20 1.6400(15), B–C26 1.6459(15), N1–N2 1.3627(12),
N1–C3 1.3644(13), N2–C1 1.3338(13); C12–B–N1 105.46(8), C12–B–
C20 107.90(8), N1–B–C20 110.10(8), C12–B–C26 104.26(8), N1–B–
C26 111.85(8), C20–B–C26 116.44(8), C13–C12–B 176.25(11), C12–
C13–C14 178.75(11).
requires only small activation energies, as experimentally evi-
denced by the ease of formation of the adducts 4–10. CO2
fixation also proved to proceed in a strongly exothermic manner,
affording a highly stable adduct 3, which showed no release of
CO2. Therefore, fine-tuning and balancing of the Lewis acidity
and basicity of pyrazolylboranes related to 1 might lead to less
strong and consequently reversible CO2 binding. In combination
with dihydrogen activation,17 the design of metal-free catalysts
for CO2 hydrogenation can be envisaged.27–29
Scheme 6 C–H activation and phenylacetylene fixation by FLP 1.
1,2-addition with formation of zwitterionic alkenes.12c,24b,34
Thus, the reactivity of 1 towards phenylacetylene was studied by
stirring a solution of both compounds in toluene for two hours at
1
ambient temperature. The H NMR spectrum of the white solid
that was obtained in 84% yield showed a broad low-field reson-
ance at 12.20 ppm, indicating the formation of the protonated
pyrazolium species 10 by heterolytic C–H bond activation
(Scheme 6). Similar reactivity was observed for intramolecular
P/B and N/B FLP systems,12c,18 which exhibit high-field 11B
NMR resonances in a similar range as found for 10 (−8.9 ppm),
suggesting the presence of an alkynyl borate moiety. This struc-
tural assignment was confirmed by X-ray diffraction analysis
of 10·toluene (Fig. 9), and the linearly bound PhCC moiety
(B–C12–C13 = 176.25(11)°, C12–C13–C14 = 178.75(11)° dis-
plays a B–C12 bond length of 1.5921(15) Å, which is similar to
that established for related phenylalkynylborate systems.8,12c,35
Experimental section
General procedures
All operations with air and moisture-sensitive compounds were
performed in a glove box under a dry argon atmosphere
(MBraun 200B) or on a high vacuum line using Schlenk tech-
niques. Carbon dioxide gas was purchased from Westfalen AG
and passed through a P4O10 column prior to use. All solvents
were purified by a solvent purification system from MBraun and
stored over molecular sieves (4 Å). The H, 13C, 11B, and 19F
1
NMR spectra were recorded on Bruker DPX 200, Bruker AV
300 and Bruker DRX 400 spectrometers. Tetramethylsilane
(TMS) was used as internal standard for H and 13C. Trichloro-
1
Conclusions
fluoromethane (CFCl3) and boron trifluoride diethyletherate
(BF3·OEt2) were used as external reference for the 19F and 11B
NMR spectra, respectively. Chemical shifts are reported in ppm
(parts per million). Coupling constants (J) are reported in Hertz
(Hz), and splitting patterns are indicated as s (singlet),
d (doublet), t (triplet), q (quartet), m (multiplet), sept (septet)
and br (broad). Elemental analysis was carried out with a Vario
Micro Cube System. A Bruker Vertex 70 spectrometer was used
for recording the IR spectra. Unless otherwise indicated, all start-
ing materials were obtained from Aldrich and were used without
The bifunctional frustrated pyrazolylborane Lewis pair 1 was
successfully employed for the fixation of carbon dioxide and
related small molecules. In 1, the Lewis basic nitrogen atom and
the Lewis acidic boron atom are ideally oriented (or preorga-
nized)20 for the interaction with an incoming ambiphilic mol-
ecule such as CO2. A very low barrier was calculated for the
formation of the CO2 adduct 3, and it is a reasonable assumption
that the fixation of other substrates, such as formaldehyde, iso-
cyanates, isothiocyanates, nitriles, isonitriles and alkynes, also
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

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further purification. The pyrazolylborane 1 was prepared accord-
ing to a published procedure.17 All used glassware was silylated
prior to use, and toluene was dried over sodium–potassium alloy.
stirred for 3 days at room temperature. The solvent was removed
under vacuum, and the residue was washed with pentane (3 mL).
After drying under vacuum, 5 was isolated as a white powder
(254 mg, 76%). Crystals suitable for X-ray diffraction analysis
were obtained from a mixture of toluene and n-hexane at
Synthesis and characterisation of the CO2 adduct 3
1
−30 °C. H NMR (400 MHz, C6D6): δ/ppm 0.83 (s, 9H, CH3),
A toluene solution (15 mL) of 1 (182 mg, 0.35 mmol) was
purged with CO2 for 10 minutes at room temperature. The pale
yellow solution became colourless within one minute. After stir-
ring the mixture for one hour under CO2 atmosphere, the solvent
was removed under vacuum. The residue was washed with
pentane (5 mL) and dried under vacuum to yield 3 as a white
powder (179 mg, 90%). Crystals suitable for X-ray diffraction
analysis were obtained from a mixture of CH2Cl2 and n-hexane
1.38 (s, 9H, CH3), 1.44 (s, 9H, CH3), 6.07 (s, 1H, CH);
11B NMR (128 MHz, C6D6): δ/ppm 2.5 (s); 13C NMR
(100 MHz, C6D6): δ/ppm 28.3 (CH3, Me), 29.3 (CH3, Me
(tBuNCO)), 29.8 (CH3, Me), 32.8 (Cq, CMe3), 33.5 (Cq, CMe3),
54.1 (Cq, CMe3 (tBuNCO)), 109.4 (CH, CvCHC), 116.2 (Cq,
1
BCipso), 137.6 (Cq, d, JCF = 251 Hz meta-C6F5), 139.8 (Cq,
1
NCO), 141.2 (Cq, d, JCF = 248 Hz para-C6F5), 148.9 (Cq, d,
1JCF = 241 Hz, ortho-C6F5), 155.5 (Cq, NvCCCH), 158.3 (Cq,
NvCCCH); 19F NMR (376 MHz, C6D6): δ/ppm −136.0 (m, 4F,
1
at −30 °C. H NMR (400 MHz, CD2Cl2): δ/ppm 1.06 (s, 9H,
CH3), 1.45 (s, 9H, CH3), 6.41 (s, 1H, CH); 11B NMR
(128 MHz, CD2Cl2): δ/ppm 2.3 (s); 13C NMR (100 MHz,
CD2Cl2): δ/ppm 28.6 (CH3, Me), 29.6 (CH3, Me), 33.5 (Cq,
CMe3), 33.8 (Cq, CMe3), 111.1 (CH, CvCHC), 137.8 (Cq, d,
o-C6F5), −154.8 (m, JFF = 21.0 Hz, 2F, p-C6F5), −163.4 (m,
3
4F, m-C6F5); elemental analysis (%) calculated for C28H28-
BF10N3O: C 54.0, H 4.5, N 6.7, found: C 54.1, H 4.6, N 6.6.
1
1JCF = 250 Hz meta-C6F5), 141.7 (Cq, d, JCF = 256 Hz para-
Synthesis and characterisation of the tBuNCS adduct 6
1
C6F5), 144.6 (Cq, NCOO), 148.8 (Cq, d, JCF = 252 Hz, ortho-
C6F5), 157.9 (Cq, NvCCCH), 162.5 (Cq, NvCCCH); 19F
A toluene solution (5 mL) of 1 (250 mg, 0.48 mmol) was treated
with tert-butyl isothiocyanate (53 mg, 0.55 mmol). The pale
yellow solution became colourless within 15 minutes, and the
reaction mixture was stirred for one day at room temperature.
After evaporation of the solvent, the residue was washed with
pentane (5 mL). The remaining white solid was dried under
vacuum to afford 6 in almost quantitative yield (298 mg, 97%).
Crystals suitable for X-ray diffraction analysis were obtained
3
NMR (376 MHz, CD2Cl2): δ/ppm −135.5 (m, JFF = 18.7 Hz,
3
4F, o-C6F5), −154.9 (m, JFF = 20.4 Hz, 2F, p-C6F5), −163.5
(m, 4F, m-C6F5); elemental analysis (%) calculated for
C24H19BF10N2O2: C 50.7, H 3.3, N 4.9, found: C 50.5, H 3.6,
N 4.8. IR (ATR): 1/λ/cm−1 2979 (w), 1803 (s), 1647 (w),
1519 (s), 1462 (s), 1106 (s), 958 (s).
1
from a mixture of toluene and n-hexane at −30 °C. H NMR
Synthesis and characterisation of the CH2O adduct 4
(400 MHz, C6D6): δ/ppm 0.82 (s, 9H, CH3), 1.33 (s, 9H, CH3),
1.39 (s, 9H, CH3), 6.24 (s, 1H, CH); 11B NMR (128 MHz,
C6D6): δ/ppm −3.3 (s); 13C NMR (100 MHz, C6D6): δ/ppm
28.1 (CH3, Me (tBuNCS)), 29.1 (CH3, Me), 29.6 (CH3, Me),
A toluene solution (5 mL) of 1 (250 mg, 0.48 mmol) was treated
with paraformaldehyde (14 mg, 0.48 mmol). The pale yellow
solution became colourless after stirring the mixture at room
temperature for one hour. The reaction mixture was stirred over-
night, whereupon a light precipitate formed. The solvent was
removed under vacuum, and the residue was washed with
pentane (2 mL). After drying under vacuum, 4 was isolated as a
white powder in almost quantitative yield (260 mg, 97%). Crys-
tals suitable for X-ray diffraction analysis were obtained from a
mixture of toluene and n-hexane at −30 °C. 1H NMR
(400 MHz, CD2Cl2): δ/ppm 0.97 (s, 9H, CH3), 1.29 (s, 9H,
CH3), 5.56 (s, 2H, CH2), 6.19 (s, 1H, CH); 11B NMR
(128 MHz, CD2Cl2): δ/ppm 2.5 (s); 13C NMR (100 MHz,
CD2Cl2): δ/ppm 29.1 (CH3, Me), 29.8 (CH3, Me), 31.9 (Cq,
CMe3), 32.9 (Cq, CMe3), 78.6 (CH2, NCH2O), 107.3 (CH,
33.4 (Cq, CMe3), 34.4 (Cq, CMe3), 56.4 (Cq, CMe3 (tBuNCS)),
1
111.1 (CH, CvCHC), 119.0 (Cq, BCipso), 137.7 (Cq, d, JCF
=
1
253 Hz meta-C6F5), 140.9 (Cq, d, JCF = 255 Hz para-C6F5),
146.2 (Cq, NCS), 145.8 (Cq, d, JCF = 253 Hz, ortho-C6F5),
1
156.1 (Cq, NvCCCH), 160.5 (Cq, NvCCCH); 19F NMR
(376 MHz, C6D6): δ/ppm −130.9 (m, 4F, o-C6F5), −156.1
(m, 2F, p-C6F5), −163.5 (m, 4F, m-C6F5); elemental analysis (%)
calculated for C28H28BF10N3S: C 52.6, H 4.4, N 6.6, found:
C 52.3, H 4.6, N 6.2.
Synthesis and characterisation of the MeNCS adduct 7
1
CvCHC), 116.9 (Cq, BCipso), 137.5 (Cq, d, JCF = 250 Hz
A toluene solution (5 mL) of 1 (200 mg, 0.38 mmol) was treated
with methyl isothiocyanate (28 mg, 0.38 mmol). The mixture
was stirred for two days at room temperature. After evaporation
of the solvent, the residue was washed with pentane (2 mL). The
remaining white solid was dried in vacuum to afford 7 (210 mg,
92%). Crystals suitable for X-ray diffraction analysis were
1
meta-C6F5), 140.9 (Cq, d, JCF = 252 Hz para-C6F5), 149.0
1
(Cq, d, JCF = 243 Hz, ortho-C6F5), 151.2 (Cq, NvCCCH),
156.8 (Cq, NvCCCH); 19F NMR (376 MHz, CD2Cl2): δ/ppm
3
−135.3 (m, 4F, o-C6F5), −157.6 (m, JFF = 20.5 Hz, 2F,
p-C6F5), −164.7 (m, 4F, m-C6F5); elemental analysis (%) calcu-
lated for C24H21BF10N2O: C 52.0, H 3.8, N 5.1, found: C 52.3,
H 4.1, N 4.9.
1
obtained from a mixture of toluene and pentane at −30 °C. H
NMR (300 MHz, C6D6): δ/ppm 0.71 (s, 9H, CH3), 1.63 (s, 9H,
CH3), 3.09 (s, 3H, CH3), 6.10 (s, 1H, CH); 11B NMR (96 MHz,
C6D6): δ/ppm −1.8 (s); 13C NMR (75 MHz, C6D6): δ/ppm 28.8
(CH3, Me), 29.9 (CH3, Me), 32.6 (Cq, CMe3), 33.9 (CH3, Me
(MeNCS)), 34.3 (Cq, CMe3), 110.0 (CH, CvCHC), 137.9 (Cq,
Synthesis and characterisation of the tBuNCO adduct 5
A toluene solution (5 mL) of 1 (279 mg, 0.53 mmol) was treated
with tert-butyl isocyanate (53 mg, 0.53 mmol). The mixture was
1
1
d, JCF = 251 Hz meta-C6F5), 141.3 (Cq, d, JCF = 254 Hz,
This journal is © The Royal Society of Chemistry 2012
Dalton Trans.

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1
para-C6F5), 148.8 (Cq, d, JCF = 236 Hz, ortho-C6F5), 157.8
the reaction was stirred for two hours. After evaporation of the
solvent, the residue was washed with pentane (5 mL), and the
remaining white solid was dried in vacuum (120 mg, 83%).
Crystals suitable for X-ray diffraction analysis were obtained
(Cq, NvCCCH), 159.7 (Cq, NvCCCH), 171.3 (Cq, NCS); 19F
NMR (188 MHz, C6D6): δ/ppm −134.5 (m, 4F, o-C6F5), −153.6
(m, JFF = 21.0 Hz, 2F, p-C6F5), −162.2 (m, 4F, m-C6F5);
elemental analysis (%) calculated for C25H22BF10N3S: C 50.3,
H 3.7, N 7.0, found: C 50.7, H 3.9, N 6.6.
3
1
from toluene solution at −30 °C. H NMR (400 MHz, C7D8):
4
δ/ppm 0.89 (s, 9H, CH3), 0.92 (s, 9H, CH3), 5.96 (d, JHH = 2.7
Hz 1H, CH), 6.93 (m, 3H, m- and p-Ph), 7.42 (m, 2H, o-Ph),
12.20 (br s, 1H, NH); 11B NMR (128 MHz, C7D8): δ/ppm −8.9
(s); 13C NMR (100 MHz, C7D8): δ/ppm 29.0 (CH3, Me), 29.7
(CH3, Me), 31.22 (Cq, CMe3), 33.0 (Cq, CMe3), 102.4 (Cq,
Synthesis and characterisation of the PhCN adduct 8
A toluene solution (2 mL) of 1 (92 mg, 0.18 mmol) was treated
with benzonitrile (18 mg, 0.18 mmol) at room temperature. The
pale yellow solution became immediately colourless, and the
reaction mixture was stirred for another ten minutes. After evap-
oration of all volatiles, 8 was isolated as a white solid (93 mg,
85%). Crystals suitable for X-ray diffraction analysis were
Ph-CC)), 104.7 (CH, CvCHC), 124.8 (Cq, i-Ph), 128.5 (Cq,
1
p-Ph), 128.5 (Cq, m-Ph), 131.7 (Cq, o-Ph), 137.8 (Cq, d, JCF
=
252 Hz meta-C6F5), Cq para-C6F5 is not observed, 148.9 (Cq, d,
1JCF = 239 Hz, ortho-C6F5), 153.9 (Cq, NvCCCH), 162.5 (Cq,
NvCCCH); 19F NMR (376 MHz, C7D8): δ/ppm −133.7 (m, 4F,
3
1
o-C6F5), −157.5 (m, JFF = 20.0 Hz, 2F, p-C6F5), −164.1
obtained from a toluene/n-hexane mixture at −30 °C. H NMR
(m, 4F, m-C6F5); elemental analysis (%) calculated
for C31H25BF10N2: C 59.5, H 4.0, N 4.5, found: C 59.3, H 4.2,
N 4.4.
(400 MHz, C6D6): δ/ppm 0.79 (br s, 9H, CH3), 0.94 (br s, 9H,
CH3), 6.08 (s, 1H, CH), 6.96 (m, 3H, m- and p-Ph), 7.37 (m,
2H, o-Ph); 11B NMR (128 MHz, C6D6): δ/ppm 1.9 (s); 13C
NMR (100 MHz, C6D6): δ/ppm 29.4 (br s, CH3, Me), 30.4 (br s,
CH3, Me), 31.8 (Cq, CMe3), 32.8 (br s, Cq, CMe3), 109.2 (CH,
CvCHC), 116.9 (Cq, BCipso), 128.3 (Cq, m-Ph), 129.9 (Cq,
o-Ph), 130.4 (Cq, p-Ph), 133.4 (br s, Cq, NCPh), 137.7 (Cq, d,
X-Ray diffraction studies
X-Ray diffraction analysis data were recorded at 100 K
on Oxford Diffraction diffractometers using monochromated
Mo-Kα or mirror-focussed Cu-Kα radiation. The structures were
refined anisotropically on F2 using the SHELX-97 program.36
Absorption corrections were based on multi-scans. Hydrogen
atoms were included using rigid idealized methyl groups or a
riding model. Crystallographic data are summarized in Table 1.
Exceptions and special features: compounds 4 and 9 crystallized
with a molecule of toluene disordered across an inversion centre.
For 4 this could not be satisfactorily refined, and the effects of
the solvent were therefore removed with the program
SQUEEZE;37 for 9 a disorder model involving an idealized aro-
matic ring was successfully employed. One C6F5 ring of com-
pound 8 is disordered over two positions with occupancies
0.788/0.212(4). An appropriate system of restraints was used to
improve refinement stability, but the dimensions of disordered
groups should always be interpreted with caution. For compound
10, the NH hydrogen was refined freely; the toluene molecule is
well-ordered.
1
1JCF = 251 Hz meta-C6F5), 143.0 (Cq, d, JCF = 254 Hz, para-
1
C6F5), 149.0 (Cq, d, JCF = 246 Hz, ortho-C6F5), 152.3 (br s,
Cq, NvCCCH), 160.5 (br s, Cq, NvCCCH); 19F NMR
(376 MHz, C6D6): δ/ppm −134.8 (m, 4F, o-C6F5), −156.7
(m, 2F, p-C6F5), −163.7 (m, 4F, m-C6F5); elemental analysis (%)
calculated for C30H24BF10N3: C 57.4, H 3.9, N 6.7, found:
C 57.6, H 4.2, N 6.5.
Synthesis and characterisation of the tBuNC adduct 9
A toluene solution (5 mL) of 1 (250 mg, 0.48 mmol) was treated
with tert-butyl isocyanide (40 mg, 0.48 mmol) at room tempera-
ture. The pale yellow solution became immediately colourless,
and the reaction mixture was stirred for 30 minutes. After evap-
oration of all volatiles, 9 was isolated as a white solid (277 mg,
95%). Crystals suitable for X-ray diffraction analysis were
1
obtained from toluene solution at −30 °C. H NMR (300 MHz,
C7D8): δ/ppm 0.93 (s, 9H, CH3), 0.98 (s, 9H, CH3), 1.42 (s, 9H,
CH3), 6.13 (s, 1H, CH); 11B NMR (96 MHz, C7D8): δ/ppm −9.0
(s); 13C NMR (75 MHz, C7D8): δ/ppm 28.3 (CH3, Me), 30.7
(CH3, Me), 31.2 (CH3, Me), 31.8 (Cq, CMe3), 32.4 (Cq, CMe3),
59.3 (Cq, CMe3 (tBuNC)), 102.0 (CH, CvCHC), 138.3 (Cq, d,
1JCF = 258 Hz meta-C6F5), Cq para-C6F5 is not observed, 149.0
Theoretical calculations
The calculations were performed using the GAUSSIAN09
package.38 All structures were fully optimized on the density
functional theory (DFT) level employing the M06-2X func-
tional.39 For all elements (C, H, B, N and F) the all-electron
triple-ζ basis set (6-311++G**) was used.40 Additional details of
the electronic structure calculations and presentations of all cal-
culated structures together with Cartesian coordinates of their
atomic positions are provided as ESI.†
1
(Cq, d, JCF = 244 Hz, ortho-C6F5), 157.5 (Cq, NvCCCH),
160.6 (Cq, NvCCCH); 19F NMR (188 MHz, C7D8): δ/ppm
3
−133.3 (m, 4F, o-C6F5), −155.4 (m, JFF = 20.5 Hz, 2F,
p-C6F5), −163.5 (m, 4F, m-C6F5); elemental analysis (%) calcu-
lated for C28H28BF10N3: C 55.4, H 4.7, N 6.9, found: C 55.3,
H 4.5, N 7.1.
Synthesis and characterisation of the PhCuCH adduct 10
Acknowledgements
A toluene solution (5 mL) of 1 (118 mg, 0.23 mmol) was treated
with phenylacetylene (23 mg, 0.23 mmol) at room temperature.
The pale yellow solution became immediately colourless, and
This work is supported by the Deutsche Forschungsgemeinschaft
(DFG) through grant Ta 189/9-1 and by the Deutsche
Dalton Trans.
This journal is © The Royal Society of Chemistry 2012

Table 1 Crystallographic data
3
4·1/2(toluene)
5
6
7
8
9·1/2(toluene)
10·toluene
Empirical formula
Formula weight
T/K
C24H19BF10N2O2
568.22
100(2)
C27.5H25BF10N2O
600.30
100(2)
C28H28BF10N3O
623.34
100(2)
C28H28BF10N3S
639.40
100(2)
C25H22BF10N3S
597.33
100(2)
C30H24BF10N3
627.33
100(2)
C31.5H32BF10N3
653.41
100(2)
C38H33BF10N2
718.47
100(2)
Wavelength λ/Å
Crystal system
Space group
0.71073
Monoclinic
C2/c
1.54184
0.71073
Monoclinic
P21/n
1.54184
Monoclinic
P21/n
1.54184
Monoclinic
P21/c
0.71073
Monoclinic
P21/n
0.71073
1.54184
Triclinic
Triclinic
Triclinic
ˉ
ˉ
ˉ
P1
P1
P1
a/Å
b/Å
c/Å
α (°)
β (°)
γ (°)
Volume [Å3]
Z
15.3482(4)
14.1117(4)
23.0099(6)
90
108.656(4)
90
9.2377(6)
11.5573(10)
13.2013(12)
73.287(8)
86.450(6)
83.350(6)
1340.18(19)
2
10.9606(4)
19.1668(6)
14.3458(4)
90
102.930(4)
90
11.1232(2)
13.5996(2)
19.3628(2)
90
96.913(2)
90
10.1128(2)
12.5196(2)
20.7678(4)
90
97.885(2)
90
11.8154(3)
15.1224(5)
15.9346(5)
90
97.556(3)
90
9.5902(12)
11.3185(16)
15.1442(18)
85.961(10)
88.454(10)
74.367(12)
1586.5(4)
2
11.0480(6)
13.2011(7)
13.2014(7)
73.524(5)
69.985(5)
78.473(5)
1723.48(16)
2
4721.8(2)
8
2937.34(16)
4
2907.4(7)
4
2604.51(8)
4
2822.43(15)
4
Reflections collected 95 441
14 213
5492
[Rint = 0.0181]
1.488
1.218
106 116
7000
[Rint = 0.0554]
1.410
54 092
5941
[Rint = 0.0272]
1.461
1.795
41 673
5324
[Rint = 0.0289]
1.523
1.962
67 513
4972
[Rint = 0.0515]
1.476
0.133
36 032
6468
[Rint = 0.0827]
1.368
0.121
63 138
7144
[Rint = 0.0222]
1.384
1.022
Independent
reflections
ρC/g cm−3
μ/mm−1
4817
[Rint = 0.0475]
1.599
0.155
0.130
R(FO), [I > 2σ(I)]
0.0322
0.0740
1.011
0.0332
0.0929
1.051
0.0426
0.0983
1.028
0.0283
0.0735
1.048
0.0289
0.0784
1.037
0.0371
0.0854
1.034
0.0506
0.1187
1.029
0.0326
0.0849
1.038
2
Rw (FO
)
Goodness of fit on
(F2)
Δρ/eÅ−3
0.297/−0.223
0.241/−0.197
0.321/−0.300
0.285/−0.222
0.291/−0.210
0.298/−0.244
0.251/−0.241
0.378/−0.258

View Online
K. Bergander, B. Wibbeling, R. Fröhlich and G. Erker, Dalton Trans.,
2009, 1534.
Bundesstiftung Umwelt (DBU) through a scholarship for
E. Theuergarten.
14 (a) V. Sumerin, F. Schulz, M. Nieger, C. Wang, M. Leskelä, P. Pyykkö,
T. Repo and B. Rieger, J. Organomet. Chem., 2009, 694, 2654;
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15 D. W. Stephan, S. Greenberg, T. W. Graham, P. Chase, J. J. Hastie,
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Notes and references
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16 G. Lu, H. Li, L. Zhao, F. Huang and Z.-X. Wang, Inorg. Chem., 2010,
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2 G. C. Welch, R. R. San Juan, J. D. Masuda and D. W. Stephan, Science,
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17 E. Theuergarten, D. Schlüns, J. Grunenberg, C. G. Daniliuc, P. G. Jones
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18 C. M. Mömming, S. Frömel, G. Kehr, R. Fröhlich, S. Grimme and
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19 C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich, B. Schirmer,
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20 F. Bertini, V. Lyaskovskyy, B. J. J. Timmer, F. J. J. de Kanter, M. Lutz,
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21 C. M. Mömming, E. Otten, G. Kehr, R. Fröhlich, S. Grimme,
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22 C. M. Mömming, G. Kehr, B. Wibbeling, R. Fröhlich and G. Erker,
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3 Selected articles: (a) G. C. Welch and D. W. Stephan, J. Am. Chem. Soc.,
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Ed., 2008, 47, 38; (c) D. P. Huber, G. Kehr, K. Bergander, R. Fröhlich,
G. Erker, S. Tanino, Y. Ohki and K. Tatsumi, Organometallics, 2008, 27,
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4 (a) H. Wang, R. Fröhlich, G. Kehr and G. Erker, Chem. Commun., 2008,
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23 S. Porcel, G. Bouhadir, N. Saffon, L. Maron and D. Bourissou, Angew.
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24 (a) C. Rosorius, G. Kehr, R. Fröhlich, S. Grimme and G. Erker, Organo-
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25 (a) J. Boudreau, M.-A. Courtemanche and F.-G. Fontaine, Chem.
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5 (a) C. Jiang, O. Blacque and H. Berke, Organometallics, 2009, 28, 5233;
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26 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2010, 132,
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27 A. E. Ashley, A. L. Thompson and D. O`Hare, Angew. Chem., Int. Ed.,
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28 A. Berkefeld, W. E. Piers and M. Parvez, J. Am. Chem. Soc., 2010, 132,
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29 S. D. Tran, T. A. Tronic, W. Kaminsky, D. M. Heinekey and J. M. Mayer,
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32 (a) M. Tamm and F. E. Hahn, Coord. Chem. Rev., 1999, 182, 175;
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33 M. Tamm, T. Lügger and F. E. Hahn, Organometallics, 1996, 15, 1251.
34 H. Jacobsen, H. Berke, S. Döring, G. Kehr, G. Erker, R. Fröhlich and
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35 M. A. Dureen and D. W. Stephan, J. Am. Chem. Soc., 2009, 131, 8396.
36 G. M. Sheldrick, Acta Crystallogr., Sect. A: Found. Crystallogr., 2008,
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6 (a) D. Holschumacher, T. Bannenberg, C. G. Hrib, P. G. Jones and
M. Tamm, Angew. Chem., Int. Ed., 2008, 47, 7428;
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(d) S. Kronig, E. Theuergarten, C. G. Daniliuc, P. G. Jones and
M. Tamm, Angew. Chem., Int. Ed., 2012, 51, 3240.
7 (a) P. A. Chase and D. W. Stephan, Angew. Chem., Int. Ed., 2008, 47,
7433; (b) P. A. Chase, A. L. Gille, T. M. Gille and D. W. Stephan, Dalton
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8 M. Alcarazo, C. Gomez, S. Holle and R. Goddard, Angew. Chem., Int.
Ed., 2010, 49, 5788.
9 (a) G. Ménard and D. W. Stephan, J. Am. Chem. Soc., 2010, 132, 1796;
(b) Y. Zhang, G. M. Miyake and E. Y.-X. Chen, Angew. Chem., Int. Ed.,
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10 A. Schäfer, M. Reißmann, A. Schäfer, W. Saak, D. Haase and T. Müller,
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37 A. L. Spek, SQUEEZE forms part of the PLATON program suite, Univer-
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11 A. M. Chapman, M. F. Haddow and D. F. Wass, J. Am. Chem. Soc.,
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38 M. J. Frisch, GAUSSIAN 09 (Revision A.1), Gaussian, Inc., Wallingford,
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39 (a) Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215;
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40 R. Krishnan, J. S. Binkley, R. Seeger and J. A. Pople, J. Chem. Phys.,
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12 (a) K. V. Axenov, C. M. Mömming, G. Kehr, R. Fröhlich and G. Erker,
Chem.–Eur. J., 2010, 16, 14069; (b) Z. M. Heiden, M. Schedler and
D. W. Stephan, Inorg. Chem., 2011, 50, 1470; (c) S. Schwendemann,
R. Fröhlich, G. Kehr and G. Erker, Chem. Sci., 2011, 2, 1842.
13 (a) P. Spies, G. Erker, G. Kehr, K. Bergander, R. Fröhlich, S. Grimme
and D. W. Stephan, Chem. Commun., 2007, 5072; (b) P. Spies, G. Kehr,
Dalton Trans.
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